1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with fuel assemblies positioned adjacent the baffle structure about the periphery of the reactor core, which employ anti-vibration grids.
2. Related Art
A typical nuclear power reactor includes a reactor vessel housing a nuclear reactor core. Spaced radially, inwardly from the reactor vessel is a generally cylindrical core barrel and within the barrel is a former and a baffle system (hereafter called the “baffle structure”), which permits transition from the cylindrical barrel to a squared-off periphery of the reactor core formed by the fuel assemblies arrayed therein.
The reactor core is composed of a large number of elongated fuel assemblies. Each fuel assembly includes a plurality of fuel rods containing the fissile material, which reacts to produce heat. The fuel rods of each fuel assembly are held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly.
During operation of the reactor, a coolant fluid such as water is typically pumped into the reactor vessel through a plurality of inlet nozzles. The coolant fluid passes downward through an annular region defined between the reactor vessel and the core barrel, turns in a lower plenum defined in the reactor vessel, then passes upwardly through the fuel assemblies of the reactor core, and exits from the vessel through a plurality of outlet nozzles extending through the core barrel. Heat energy, which the fuel rods of the fuel assemblies impart to the coolant fluid, is carried off by the fluid from the vessel. Due to the existence of holes in the core barrel, coolant fluid is also present between the barrel and a baffle structure and at a higher pressure than within the core. However the baffle structure, together with the core barrel, do separate the coolant fluid from the fuel assemblies as the fluid flows downwardly through the annular region between the reactor vessel and core barrel.
As mentioned above, the baffle structure surrounds the fuel assemblies of the reactor core. Typically, the baffle structure is made of plates joined together by bolts. These bolts sometimes become loose, thereby developing a small gap between the baffle structure plates. When this happens, a coolant fluid jetting action takes place through the baffle structure in a radially inward direction from the exterior of the core to the interior thereof, due to the greater fluid pressure existing outside of the baffle than within the core. In some reactors, the baffle structure contains slots and holes intentionally placed to allow cooling of the core during an accident condition. As with the gaps that open due to loose bolts, the coolant flow enters the core through the baffle slots and holes and causes fluid jetting. The fluid jetting, when it impinges on the outer row of fuel assemblies in the core, makes the outer fuel rods vibrate, eventually causing them to fail. Consequently, the need exists for a way to deal effectively with the fluid jetting through portions of the baffle structure so as to avoid its deleterious affects on the fuel rods of the fuel assemblies positioned adjacent the baffle structure.